Method for use of static inverters in variable energy generation environments

ABSTRACT

A method to collect energy from generation systems such as, for example, wind farms or solar farms with widely distributed energy-generation equipment. In some cases, static inverters are used to feed the energy directly into the power grid. In some other cases, back-to-back static inverters are used create a high-voltage DC transmission line to collect power from multiple generation sites into one feed-in site.

RELATED APPLICATION

The present application is a non-provisional application which claimsthe benefit of U.S. Provisional Patent Application Ser. No. 61/397,320entitled “System and Method for Use of Static Inverters in VariableEnergy Generation Environments” filed Jun. 9, 2010, which is herebyincorporated by reference in its entirety.

COPYRIGHT NOTICE AND PERMISSION

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the patent and trademarkoffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF INVENTION

Embodiments of this invention include methods for collecting energy fromvariable energy generation systems for transmission.

BACKGROUND

In variable energy generation systems, such as wind, solar, and otheropportunistic power generation systems, the amount of available energyat any given time is not known. Also, these systems are often physicallydistributed over a large area, thus creating a challenge for collectingthe energy with minimum power losses.

BRIEF SUMMARY OF THE INVENTION

Embodiments of this invention include methods to collect energy fromgeneration systems such as, for example, wind farms or solar farms withwidely distributed energy-generation equipment. In some cases, staticinverters are used to feed the energy directly into the power grid. Inother cases, back-to-back static inverters are used to create ahigh-voltage DC transmission line to collect power from multiplegeneration sites into one feed-in site.

These and other objects of the present invention will become clear tothose skilled in the art in view of the description of the bestpresently known mode of carrying out the invention and the industrialapplicability of the preferred embodiment as described herein and asillustrated in the figures of the drawings. The embodiments areillustrated by way of example and not limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

The purposes of the present invention will be apparent from thefollowing detailed description in conjunction with the appended figuresof drawings, in which:

FIG. 1 shows an embodiment of the present invention employing a variableenergy generation system with a static inverter.

FIG. 2 shows a six-phase star circuit.

FIG. 3 a shows a delta-wye type of transformer.

FIG. 3 b shows an alternative design of a static inverter or rectifieraccording to another aspect of the method disclosed herein.

FIG. 4 a shows another exemplary simplified static inverter or rectifierin a three-phase full-wave bridge circuit, according to one aspect ofthe method disclosed herein.

FIG. 4 b shows voltage waveforms with output voltage and phase voltages.

FIG. 5 a shows a balanced inter-reactor system with a delta-wye-wyetransformer and a balanced reactor on a separate core.

FIG. 5 b shows waveforms that result from a 12-pulse approach.

FIG. 6 a shows a delta-wye-delta serial configuration of a staticinverter.

FIG. 6 b shows a configuration of a static inverter/rectifier.

FIG. 7 shows an embodiment of the present invention employing a variableenergy generation system with a combination of a static inverter and apulse width modulation inverter.

FIG. 8 shows an embodiment of the present invention employing a variableenergy generation system with multiple static inverters.

In the various figures of the drawings, like references are used todenote like or similar elements or steps.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description. References to one or an embodiment in thepresent disclosure are not necessarily references to the sameembodiment; and, such references mean at least one.

The use of headings herein are merely provided for ease of reference,and shall not be interpreted in any way to limit this disclosure or thefollowing claims.

Reference in this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the disclosure. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but not other embodiments.

FIG. 1 shows an overview of an exemplary multi-point power generationsystem 100, employed by one aspect of the method disclosed herein. Shownare string sets 101 a . . . n, each equipped with a set of energycollecting units (“collectors”) 110 aa . . . nn which output directcurrent (DC). At the end of each string is a string converter 111 a . .. n that feeds high-voltage into a floating DC bus, typically, forexample, in a range between 100 volts and 1000 volts. Some regulatorybodies place limits on the voltages, such as between 50 and 600 volts,in some cases as high as 1000 volts, but for purposes of thisdiscussion, the actual values of these local regulatory limits are notimportant.

One of the negative aspects of using a static inverter is that the inputvoltage is transformed at a given ratio into the output voltage. Thus,an input voltage set at, for example, 500 volts, results in a specificAC power at a certain voltage. To feed properly into the grid, thevoltage and the phase is adjusted. The phase is easily adjusted bycontrolling the timing of the switches used in the static inverter.However, in normal operation, the voltage is not easily adjustable. Inthe exemplary system 100 of FIG. 1, the string converters 111 a . . . nare used to move the floating DC bus up or down according to the currentenergy production, so that static inverter 120 with its fixed ratio cangenerate the correct voltage to feed into the grid.

Static inverters have several properties that can be used for advantage,although in many situations, they also can be problematic. One of theadvantages is that switching losses are substantially lower, asfrequencies are much lower, generally (range of 50-400 Hz typically).The disadvantage is that transformers can be larger. In the case ofsolar installations the transformer is typically required for systemsizes above a power rating of about 20 kW (as per today's pendingregulations, but a limit will likely be in most cases) as a result ofthe need to have a galvanic isolation between the grid and the DC bus. Atransformer is required because solar panels have leakage current atnormal operating conditions, as do, in some cases, inverters. The largerthe system, the larger and potentially more dangerous are such leakages.

Further, standard Pulse Width Modulation (PWM) inverters typically haveadditional filtering to avoid heating the transformer at the switchingfrequency, because such inverters are less efficient when driving atransformer directly. These losses are in addition to their switchinglosses. They can be operated both ways, as converters and as rectifiers(hence inverter), and finally, they have a built-in ratio between inputand output voltage that cannot be easily changed. The last point isoften a problem, but in the examples discussed herein, that problem isnot very critical, as the DC voltage bus can be adjusted by primaryinverters to provide the desired or needed voltage to feed into thegrid. Lastly, when used to feed into the grid, they have a power factorof typically 0.97 or even higher if a system with more than 12 pulses isused, but that factor can be adjusted as described below.

The aforementioned generation of the correct voltage is done with thehelp of controller 124, which has connections 125 to the stringconverters, setting the voltage outputs they need to generate. Further,controller 124 controls switches 123 a . . . n with appropriatelyinsulated drivers (typically driver transformers or optically coupledswitches, or both, or other suitable solutions) through control line 126(drivers not shown). Said line 126 is shown here simplified as one line,whereas in reality, line 126 would contain at least a separate controlline or pair for each switch, and each line would have a potentialseparator. Additionally, connection 127 connects to the grid to measurethe voltage phase, to ensure that the voltage feed is correct. Alsoshown is data connection 128, which connection could connect via theInternet or some other public or private network to the electricutility, sending real-time data about energy being delivered, as well asto a supervisory site that could control multiple power generationsites.

Table 1, below, shows some aspects of a standard PWM inverter for solarapplication as compared to the new proposed method using a staticinverter solution.

TABLE 1 Standard solution New proposed method Parameter with PWMinverter using static inverter Line transformer Mandatory aboveMandatory above 20 kw, need 20 kw but likely always using a transformerDC bus losses At full rated load DC DC bus voltage is at its bus voltageis minimal maximum level at full yielding maximum rated load yieldinglower losses at this point conduction losses by 44% (out of the typical1.3% of conduction losses Reliability Components switched Low switchingfrequency. at relatively high Inverter efficiency higher frequency bymore than 1% resulting in lower operation temperature. Aluminumelectrolytic not required. EMI Mainly affected by Low frequencycomponent switching frequency only Local MPPT for None Full solutionsolving all maximum energy mismatch conditions as result harvesting ofthermal, aging, soiling initial tolerances, shade. Price for local Needseparate AC Local MPPT by simple stage MPPT inverter per each and simpleDC-AC inverter power segment lowest price possible Cooling Need separatefans Transformer and switches can operate with natural convectioncooling.

FIG. 2 shows another approach to using a static inverter, according toone aspect of the method described herein. In this approach, a six-phasestar circuit 200 has, instead of diodes 202 a . . . n shown in thefigure, switches to generate the alternating current. The advantage ofsuch an approach is that only one switch is in series, hence reducingconduction losses. However the transformer 201 is more complicated, withadditional windings 201 d . . . i on one side (double wye), and aregular delta with three windings 201 a . . . c on the other side (AC).

FIG. 3 a shows a typical delta (304 b)-wye (304 a) type of transformer(ac winding not shown here) in static inverter or rectifier 301. In thisexample, diodes 305 a . . . f and 306 a . . . f are shown for operationin a rectifying mode, feeding through a balance transformer 303 into aload 302. In other cases, if the load is replaced with a DC bus and thediodes are replaced with switches such as, for example, FETs, SCRs,IGBTs etc, this topology could be used to both rectify or up-convert.

FIG. 3 b shows an alternative design of static inverter or rectifier310, employed by another aspect of the method disclosed herein. Staticinverter or rectifier 310 does not have balancing transformer 303. Shownare delta windings 311 a and wye windings 311 b. Also shown are the twosets of switches (as discussed above) or diodes 313 a . . . f and 314 a. . . f. The DC bus or load is resistor RL 312.

FIG. 4 a shows another exemplary simplified static inverter or rectifierin three-phase full-wave bridge circuit 400, employed by one aspect ofthe method disclosed herein. Circuit 400 is a 5-pulse type staticinverter, characterized by a simpler transformer 401 (only threewindings as a delta or wye on the switches side), as opposed to the12-pulse static inverter or rectifier discussed in other sections thatrequires a total of six windings (typically as one set of three in adelta and another three in a wye). As a result, circuit 400 has astronger ripple 411 (than a 12-pulse static inverter or rectifier wouldhave), which can be seen in FIG. 4 b. Diodes 402 a . . . f are used forrectifiers, or switches would be used for static inverters. Controlledrectifiers or other suitable switches such as MOSFeT, IGBT, or evenmercury valves may be used according to the voltage being handled. Alsoshown is the DC bus or DC load 403.

FIG. 4 b shows voltage waveforms 410, with output voltage 412 and phasevoltages 413 a . . . c.

FIG. 5 a shows a balanced inter-reactor system 500 with a delta-wye-wyetransformer and a balanced reactor on a separate core. Transformer 501has an AC side delta winding 501 a and two primary windings 501 b and501 c. Windings 501 b and 501 c have different winding ratios and/orphase assignments, thus supporting creation of a 12-pulse conversionstatic inverter or rectifier. Again, instead of standard rectifiers 504a . . . c and 505 a . . . c, SCRs or other, suitable switching devicesmay be used.

FIG. 5 b shows the waveforms 510 that result from a 12-pulse approach,instead of a 6-pulse approach. Voltages are overlaid such that a verysmall ripple results with less than 3 percent load factor. In manycases, using the 12 pulse approach is sufficient filtering forconnection to a grid; however in other cases, additional correction maybe required, as discussed below in the description of FIG. 7. Thus whenoperating from AC to DC, only minimal filter capacity needs to be added,or when operating the other way, minimal power factor correction needsto be done.

FIG. 6 a shows a delta-wye-delta serial configuration of a staticinverter 600 that does not require a balancing transformer. Also, as thetwo sets of switches are in series, the operating DC voltage can beroughly twice in relation to the breakdown voltage of the switches, asin a parallel configuration. Two sets of diodes or SCRs 603 a . . . fand 604 a . . . f are in series. As a result, the voltage is split (notevenly, but typically 1:2), resulting in the desired 12-step AC voltagethat is commonly known in static inverters. Clearly visible are the ACsides of the transformer 602 with, all on the same core, delta winding602 a, the main winding 602 b, also a delta winding, and minor winding602 c, which is a wye winding. Placing the two sets of inverter switches603 a . . . f and 604 a . . . f in series obviates the necessity for abalancing transformer. Alternating current is delivered at connectionpoint 601.

FIG. 6 b shows an different view of a configuration of a staticinverter/rectifier 620. Shown is the DC bus 626, the two DC sidewindings 621 and 624, as well as AC side windings 625 (all on samecore), switches 622 a . . . l and balancing transformer 623.

FIG. 7 shows an exemplary high-level overview of a complete variable DCpower generation and AC conversion system 700, employed by one aspect ofthe method disclosed herein. Controller 704 interacts with multipleenergy-producing units 701 a . . . n such as, for example, a multi-unitsolar pole, or a windmill, to maintain the desired voltage on the bus.Also shown is an optional rotary capacitor 707, which in this case maybe some kind of a motor with a fly wheel. In such a rotary capacitor,the field current may be used to control addition or reduction of energyand thus to stabilize the bus more efficiently and/or cost effectivelyin some cases than an actual capacitor, depending on the size of thesystem. In smaller systems, typically, standard capacitors are used.Static inverter 702 inverts DC energy to three-phase power and connectsto feeding point 705, and thence to the grid. An additional pulse widthmodulation inverter (PWMI) 703 corrects the power factor error generatedby the static inverter using the 12-pulse generation method. Also, theadditional power with modulation in high-frequency inverter 703 runs onhigher frequency as it runs on lower power. In some cases, an additionalrotary capacitor or other compensation capacitor may be required at gridconnection point 707 before the energy is fed into the grid.

FIG. 8 shows a system 800 similar to system 700, wherein each energyproduction unit, such as, for example, a solar power pole, windgenerator, etc. generates a variable, controlled voltage. In exemplaryenergy production unit 801 a, two back-to-back static inverters 801 a 1and 801 a 2 convert the variable DC voltage first into an alternatingcurrent, then back into a high-voltage direct current (HVDC), used for ahigh-voltage transmission line 810. At the end of transmission line 810an additional static inverter 802 inverts the power into three-phase AC,which is then is then fed into the grid at point 803. Controller 804interfaces between the grid measurement, the master static inverter 802,and the power generation units at the far end, to balance the voltage onthe local DC buses 801 a 5 that are fed into the internal primary andsecondary static inverters 801 a 1 and 801 a 2. Additional energyproduction units 801 a . . . n could be, for example, a multi-unit solarpole, or a windmill, or any other variable-power generation unit.

The claims included herein include several embodiments. One embodimentinvolves collecting energy from variable energy sources such as solar orwind energy, by strings of collectors (for example photovoltaic cells orpanels, or wind turbines) as managed by string convertors andcontroller(s). Then compatible electrical energy is transported on a busto a static inverter including a transformer (such as a delta-wye-deltatransformer) and a balance transformer. The static inverter outputsalternating current at a given voltage. The controller(s) monitorvoltage phase on a grid and manages the static inverter so that thealternating current is compatible with grid current.

Another embodiment also involves collecting energy from variable energysources. In this instance, the topology of the system includes both astatic inverter and a pulse width modulation inverter. Current flowsthrough the static inverter and the pulse width inverter to a feedingpoint and then onto a grid.

Yet another embodiment again involves collecting energy from variableenergy sources. Current flows into a static inverter to convert directcurrent into alternating current. From that point, the alternatingcurrent flows into a second static inverter to convert the alternatingcurrent into high voltage direct current which is transported on a highvoltage direct current transmission line to a master static inverterwhich in turn converts the direct current into alternating currentsuitable for transmission via a grid.

While the particular method for the use of static inverters in variableenergy generation environments, as herein shown and described in detail,is fully capable of attaining the above-described objects of theinvention, it is to be understood that it is the presently preferredembodiment of the present invention, and is thus representative of thesubject matter which is broadly contemplated by the present invention,that the scope of the present invention fully encompasses otherembodiments which can become obvious to those skilled in the art, andthat the scope of the present invention is accordingly to be limited bynothing other than the appended claims, in which reference to an elementin the singular means “at least one.” All structural and functionalequivalents to the steps of the above-described preferred embodimentthat are known or later come to be known to those of ordinary skill inthe art are expressly incorporated herein by reference and are intendedto be encompassed by the present claims. Moreover, it is not necessaryfor a method to address each and every problem sought to be solved bythe present invention, for it to be encompassed by the present claims.Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public, regardless ofwhether the element, component, or method step is explicitly recited inthe claims.

It is claimed:
 1. A method, comprising: collecting electrical energyproduced by one or more strings of variable energy collectors via one ormore string converters managed by a first controller; routing theelectrical energy collected via the one or more string converters via afloating direct current bus to a static inverter having a fixed ratiofor converting a direct current input to an alternating current output,the static invert having a plurality of switches; converting theelectrical energy from the floating direct current bus to an alternatingcurrent via the static inverter to feed a power grid; measuring avoltage phase on the power grid; and controlling, by the firstcontroller, the one or more string converters to set a voltage on thefloating direct current bus to cause the static converter to output thealternating current in accordance with a voltage of the grid, and thetiming of the switches of the static inverter in accordance with thevoltage phase measured on the power grid.
 2. The method of claim 1,further comprising sending data via the first controller and a networkabout energy being output.
 3. The method of claim 1, further comprisingcommunicating data via the first controller with a supervisory site.